Of the people who do receive the evacuation orders and still don’t leave, there are many factors that come into play, from not having anywhere to go to not having transportation. Social pressure also comes into play; people are more likely to leave or stay depending on what their neighbors and friends are doing.
Another issue may be pets, but many evacuation centers and shelters are now pet-friendly. However, Baker’s studies have found that much more than those sorts of considerations, people don’t leave because they simply think they’ll be safe. Unfortunately, their perceptions are not always accurate. But in addition to those who think they are safe when they are not, many people whose homes are not vulnerable assume they are at risk and leave, which further clogs exit routes.
When Hurricane Rita hit Texas shortly after those residents had watched the devastation caused by Katrina, the effect was stark: “We had everyone evacuate,” either from fear of their homes being damaged or fear of losing power for an extended period of time, says Russell Henk, senior research engineer at the Texas A&M Transportation Institute. Many of those people could have sheltered in place or locally, says Henk, a member of a post-Rita task force that aimed to improve future evacuations. And the fact that they all tried to get out made it all the more difficult for those who most needed to leave.
Timing. Another major consideration in evacuations is how much advance notice to give. That’s not so easy to calculate, as it turns out.
Hurricane Floyd hit the southeastern United States in September 1999. At the time, it prompted what was considered the largest-scale evacuation in the history of the country. Areas from Florida all the way up through North Carolina were evacuated, putting two to three million people onto the roads. In several places, traffic ground to a standstill. Baker spoke to one person evacuating in South Carolina who said that he “got on the interstate spur that comes into 95 that goes into downtown Charleston. He said for three hours he could still see his house because the traffic was creeping so slowly.”
Before Floyd, transportation analysts were mostly focused on what is known as clearance time, says Baker. Clearance time is how long it theoretically would take to get everyone out of an area from the time you tell them to evacuate—based on normal traffic. “That’s important because the local officials need to know how long they can wait to tell people to start evacuating,” Baker says.